This invention relates generally to the field of printed document security, and more particularly, to authenticating originality of documents using watermarks in a printed image on the documents.
Advances in the arts of photocopying and digital image scanning and printing have made it increasingly easy to make copies of printed documents with high fidelity so that it is difficult to distinguish between an original printed document and a photocopy or scanned-and-printed copy of the original document. These advances have implications in regard to such secure documents as postage meter indicia, paper currency, and event and travel tickets. Therefore, it is desirable to provide secure documents with printed images that incorporate special features, sometimes referred to as “fragile watermarks”, wherein copying of the printed image results in changes of the feature in the copy relative to the original image in a manner that can be detected with a degree of reliability and convenience.
In the above-referenced related patent application, it was proposed to aid in detecting copying of an image by incorporating in the image a fragile watermark formed of wavepackets. As used herein and in the appended claims, a “wavepacket” refers to a periodic variation in grayscale reflectance, the period of the variation being described by a wavevector, applied to a region of an image, and includes a periodic variation based on a wavelet function as well as a periodic variation based on a sinusoidal function. As used herein and in the appended claims, a “wavevector” refers to a set of parameters that defines a wavelength and an orientation in two-dimensional space of a waveform. As described in the above-referenced patent application, a process of scanning and printing an image having such a watermark may result in a copy image in which the watermark is affected in a manner that can be detected to distinguish the copy image from an original image.
The present inventors have recognized that it may be desirable to further secure a watermark of this type from a sophisticated attack in which a malefactor attempts to reverse engineer and reconstruct the watermark and then to print counterfeit versions of the image with the reconstructed watermark. Therefore, it is an object of the present invention to provide a phase-space encoded watermark with an additional level of security that may deter, prevent and/or aid in detecting counterfeiting of the watermark.